I got the cheaper ones with Mastercraft and they really take a beating on the interstate. So I take them of until I get the to boat launch and then put them on. I could see them disintegrating after 100 miles or so.

Our boat rarely sits in the water for any length of time, so my dock lines are old 15' sections from my slalom ropes. As for the guide post covers, got the ones from mastercraftbydesign, and they are crap. The nylon is fine, but the writing on them mars the crap out of the rub rail if it comes anywhere near them when driving the boat on. As my 13 year old daughter pulls it on as well as anyone, and always touches, I wish I had bought plain nylon ones.
My $0.02, your results may vary. I also bought a set for my brother, so he may weigh in with a vastly different experience.

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I spent most of my money on booze, broads and boats. The rest I wasted. - Elmore Leonard
I had always thought that there was nothing quite so sad as an abandoned boat.-Terry Hayes

3/8 is plenty strong. I have 25' as I like to do a spring line (front-to-back) configuration when we have our week long cabin and have a slip. So just depends upon what you want to do. A little on the pricey side but three years and they are still like new.